Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell propulsion systems have also been proposed for use in vehicles as a replacement for internal combustion engines. The fuel cells generate electricity that is used to charge batteries and/or to power an electric motor. A solid-polymer-electrolyte fuel cell includes a polymer electrolyte membrane (PEM) that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, a fuel, commonly hydrogen (H2), but also either methane (CH4) or methanol (CH3OH), is supplied to the anode and an oxidant, such as oxygen (O2) is supplied to the cathode. The source of the oxygen is commonly air.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane. The electrons flow through an electrical load (such as the batteries or the electric motor) that is connected across the membrane. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+), and electrons (e−) are taken up to form water (H2O).
Fuel cell stack durability is traditionally defined in terms of the number of hours the fuel cell stack can maintain each of its fuel cells above a given voltage threshold for a required current draw. Because the fuel cells of the fuel cell stack are connected in series, an individual fuel cell voltage cannot be affected without affecting the voltage of every other fuel cell in the fuel cell stack. Furthermore, because only one anode and one cathode gas stream is fed to the entire fuel cell stack, the composition, flow rate, stoichiometry, pressure and humidification of the gas streams are also not individualized for each fuel cell in the fuel cell stack. Therefore, there are traditionally no options for independently affecting individual fuel cell voltages of the assembled fuel cell stack.